1. Field of the Invention
The present invention relates to a method and an apparatus for generating relativistic electron beams.
2. Description of the Prior Art
Relativistic electron beams (simply called REB hereinafter) comprise electrons which move at relativistic velocities. REB has attracted interest of researchers because of applicability as a high energy beam for machining of articles having high melting points, plasma generation, microwave generation and so forth. One of the future applications of REB is generation of high power electromagnetic radiations such as free electron laser, X-ray lasers. Generation of X-ray laser beams can be expected by population inversion of multi-valent ions resulting from plasma recombination or electron collision. The pulse widths of REB currently available are as long as several nano seconds, which are not sufficiently short for X-ray lasers requiring quick excitation.
As for ultraviolet laser beams or visible laser beams, on the other hand, pulses can be easily obtained on the order of pico seconds by virtue of nonlinear optical effect. The X-ray laser resonance has been realized so far only by means of high power pulse lasers. Until now, ultra-short pulses of REB has not been considered since optical effect on non-linear electron beams is very weak.
On the other hand, research has been broadly carried out for generating microwaves by the use of REB. Microwaves are generated from an REB whose electron density is modulated to produce compression waves. The compression waves and therefore microwaves emitted therefrom are amplified by adjusting the velocity of the REB and the phase velocity of microwaves. Namely, the compression waves are amplified by electromagnetic fields of the microwaves. This mechanism has been broadly utilized in a variety of microwave resonators such as klystrons, travelling-wave tubes, magnetron. High density REB is desirable for microwave generation at high output power. The coulomb repulsion between electrons in REB, however, is very strong so that it is difficult to suppress dissipation of beams and to modulate the electron density of REB by electromagnetic fields of microwaves.